Aldimines are condensation products that include primary amines and aldehydes and represent a family of substances that has been known. Upon contact with moisture, aldimines can hydrolyze to form the corresponding amines and aldehydes. Based on this characteristic, they can be used as a protected form of amines, or of aldehydes. Thus, for example, aldimines are used in polyurethane chemistry, where they are used as cross-linking agents that can be activated by moisture, so-called “blocked amines” or “latent curing agents,” for single- or two-component compositions that have isocyanate groups.
Advantages of using aldimines as latent curing agents in systems that have isocyanate groups lie, for example, in that the development of undesirable gas bubbles can be reduced or avoided, since the curing reaction with the blocked amine—in contrast to the direct reaction of isocyanates with moisture—does not run with release of carbon dioxide (CO2), and in that higher curing speeds and/or longer open times can be achieved. The use of aldimines as latent curing agents in compositions that have isocyanate groups can also cause problems, however. In the case of single-component compositions, the shelf life can be greatly limited by the presence of aldimine. Based on the aldehydes used for the production of aldimine and released again during the curing reaction, the compositions can have a very strong odor, moreover, which cannot be tolerated for many applications.
WO 2004/013088 A1 describes odorless polyaldimines, which are produced from primary polyamines and odorless aldehydes.
WO 2007/036571 A1 describes odorless aldimines that contain at least one hydroxyl, mercapto or secondary amino group, which also are available starting from odorless aldehydes. These odorless aldehydes can have a strongly softening effect in polymer compositions, for example, in polyurethane compositions, which may be undesirable. The relatively high molecular weight of the aldehydes results, moreover, in that the aldimines produced therefrom as latent curing agents may have to be used in a relatively large amount, which can make their use expensive.